Mitomycin is an established chemotherapeutic agent given for several different types of cancer, including breast, stomach, gullet and bladder cancer. The agent acts by cross-linking DNA so the cancer cells are unable to proliferate. When given intravenously to patients, common side effects due to the toxicity include fever, nausea, vomiting, bone marrow depression, and others (HARRISON'S PRINCIPLES OF INTERNAL MEDICINE, Wilson et al., Eds., 12th Editions, Part Eleven,. page 1592, 1991). Drug toxicity is not the only problem associated with chemotherapy. Another problem is drug resistance. Some tumor types, e.g., non-small cell lung cancer and colon cancer, exhibit primary resistance, i.e., absence of response on the first exposure to currently available, conventional chemotherapeutic agents. Other tumor types exhibit acquired resistance, which develops in a number of drug-sensitive tumor types. Drug resistant cancer cells demonstrate two types of acquired drug resistance; cells exhibiting single agent resistance or resistance to single class of anti-cancer drugs with the same mechanism of action. The second type involves cells broadly resistant to several or many chemically diverse anti-cancer drugs with different mechanisms of action. This second type of acquired resistance is known as multi-drug resistance.
Multi-drug resistance is also found in some tumor cells, such as renal and colon tumors, exhibiting primary resistance. That is, in contrast to an acquired multi-drug resistance, certain tumor types are non-responsive to initial treatment with many chemotherapeutic agents.
Multidrug-resistance is often associated with increased expression of a normal gene, the MDR1 gene, for a cell surface glycoprotein, P-glycoprotein, involved in drug efflux. P-glycoprotein expression correlates with a decrease in intracellular drug accumulation; that is, the P-glycoprotein acts as an energy-dependent pump or transport molecule that removes drugs from the cell, preventing the drug from accumulating in the cell.
P-glycoprotein is normally primarily expressed at epithelial and endothelial surfaces and seems to play a role in absorption and/or secretion. It is an active transporter that pumps hydrophobic drugs out of cells, reducing their cytoplasmic concentration and therefore toxicity. In normal cells, P-glycoprotein functions to eliminate toxic metabolites or xenobiotic compounds from the body (Endicott, J. and Ling, V., Annu. Rev. Biochem., 58:137-171, (1989)).
Cancers which express P-glycoprotein include cancers derived from tissues which normally express the MDR1 gene, namely cancers of the liver, colon, kidney, pancreas and adrenal. Expression of the gene is also seen during the course of chemotherapy with multidrug-resistant drugs in leukemias, lymphomas, breast and ovarian cancer, and many other cancers. These cancers initially respond to chemotherapy, but when the cancer relapses, the cancer cells frequently express more P-glycoprotein. There are cancers derived from tissues which do not normally express P-glycoprotein but in which P-glycoprotein expression increases during the development of the cancer. One example is chronic myelogenous leukemia, which when it goes into blast crisis, expresses more P-glycoprotein irrespective of the previous treatment history (Gottesman, M. M. Cancer Research, 53:747-754 (1993)).
The MDR1-encoded P-glycoprotein pump recognizes and transports many different substances, including most natural product anti-cancer drugs such as doxorubicin, daunorubicin, vinblastine, vincristine, actinomycin D, paclitaxel, teniposide and etoposide (Gottesman, M. et al., Current Opinion in Genetics and Development, 6:610-617 (1996)). More generally, the drugs often involved in multidrug-resistance are alkaloids or antibiotics of plant or fungal origin, and they include the vinca alkaloids, anthracyclines, epipodophyllotoxins and dactinomycin. Cross-resistance to alkylating agents such as melphalan, nitrogen mustard, and mitomycin C is occasionally observed (Endicott, J. and Ling, V., Annu. Rev. Biochem., 58:137-171, (1989)). Clearly, multidrug-resistance in cancer cells limits successful chemotherapy and suggests a poor patient prognosis.
Liposomes are closed lipid vesicles used for a variety of therapeutic purposes, and in particular, for carrying therapeutic agents to a target region or cell by systemic administration of liposomes. Liposomes having a surface grafted with chains of water-soluble, biocompatible polymer, in particular polyethylene glycol, have become important drug carries. These liposomes offer an extended blood circulation lifetime over liposomes lacking the polymer coating. The grafted polymer chains shield or mask the liposome, thus minimizing nonspecific interaction by plasma proteins. This in turn slows the rate at which the liposomes are cleared or eliminated in vivo since the liposome circulate unrecognized by macrophages and other cells of the reticuloendothelial system. Furthermore, due to the enhanced permeability and retention effect (Maeda H. et al., J. Controlled Release, 65(1-2):271 (2000)), the liposomes tend to accumulate in sites of damaged or expanded vasculature, e.g., tumors, sites of inflammation.
An extended blood circulation time is often desired to allow systemically administered liposomes to reach a target region, cell or site. For example, a blood circulation lifetime of greater than about 12 hours is preferred for liposomal-therapy to a tumor region, as the liposomes must systemically distribute and then extravasate into the tumor region.
It would be desirable to provide a formulation of mitomycin C that can be taken up by multi-drug resistant cells. It would also be desirable to formulate a liposome composition having a long blood circulation lifetime and capable of retaining an entrapped drug for a desired time, yet able to release the drug on demand. It would also be desirable to provide a formulation of mitomycin C that is as efficacious as the drug in free form, yet has a reduced systemic toxicity. Furthermore, it would be desirable to release the cytotoxic mitomycin C in response to the endogenous conditions in the tumor.